1. Field of the Invention
The present invention relates to a magneto-impedance element comprising an alloy which comprises at least one element selected from the group consisting of Fe, Co and Ni, and has a mixed texture of an amorphous phase and a crystalline phase. The present invention also relates to an azimuth sensor for detecting the direction of the geomagnetic flux, an autocanceler used in CRT displays, and a magnetic head using the magneto-impedance element.
2. Description of the Related Art
With rapid progress in development of information devices, gauging devices, and control devices, magneto-detective elements, which have a smaller size, higher sensitivity and more rapid response than conventional magnetic flux-detecting elements, have been required. Elements having a magneto-impedance effect, i.e., magneto-impedance elements (hereinafter referred to as MI elements) have attracted attention.
The magneto-impedance effect indicates a phenomenon causing a change in impedance in, for example, a closed circuit as shown in FIG. 17. When an alternating current Iac of a MHz band is applied to a wire or ribbon magnetic material Mi through an electrical power source Eac while a very weak external magnetic field Hex of several gausses is applied in the longitudinal direction of the magnetic material Mi, a voltage Emi by an impedance inherent in the magnetic material occurs between two ends of the magnetic material Mi, and its amplitude varies within a range of several tens of percent in response to the intensity of the external magnetic field Hex. Since the MI element is sensitive to an external magnetic field in the longitudinal direction of the element, the sensitivity for detecting a magnetic field does not deteriorate when the length of the sensor head is considerably short, i.e., 1 mm or less. The MI element enables fabrication of a very weak magnetic field sensor having a high resolution of 10.sup.-5 Oe or more and excitation at several MHz or more. Thus, a high-frequency magnetic field of 200 MHz to 300 MHz can be used as a carrier for frequency modulation, and thus the cutoff frequency of the magnetic field sensor can be easily set to 10 MHz or more. Accordingly, the MI element is expected to be used in novel ultra-compact magnetic heads and sensors for very weak magnetic fields.
Known materials having MI effects include, for example, amorphous ribbons of Fe--Si--B system alloys, e.g. Fe.sub.78 Si.sub.9 B.sub.13, and amorphous wires of Fe--Co--Si--B system alloys, e.g., (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 (Kaneo Mouri, et al., "Magneto-Impedance (MI) Elements", Papers of Technical Meeting on Magnetics, MAG-94 (1994), Vol. 1, No. 75-84, pp. 27-36, Institute of Electrical Engineering of Japan (IEE JAPAN)).
The Fe--Si--B system and Fe--Co--Si--B system alloys have problems when they are used as MI elements. As shown in FIG. 18, when an output voltage Emi (mV) to a positive or negative magnetic field is measured, the Fe--Si--B system alloy has low sensitivity for detecting the magnetic field as shown by a curve A, and thus a high amplification factor of about 100 times is required. The element, therefore, cannot be used as a magnetic field sensor with a high sensitivity because of noise generation. On the other hand, although the Fe--Co--Si--B system alloy shown by a curve B has a sufficiently high sensitivity, as shown in FIG. 18, it has a steep increase in the sensitivity within a range from -2 Oe to +2 Oe. As a result, it cannot be used as a sensing element for a very weak magnetic field due to non-quantitative characteristics within the range. Although it can be used in magnetic field regions of 2 Oe or more as the absolute value, a coil must be provided to apply a considerable amount of current that is required for such a large bias magnetic field.
Azimuth sensors can measure the direction of the magnetic flux of an external magnetic field such as geomagnetism, and have been widely used as sensors for vehicle compasses and navigation systems that detect the location of vehicles.
Among the azimuth sensors, since a flux gate sensor shows excellent stability according to its operational principle and a high sensitivity of 10.sup.-7 to 10.sup.-6 G, it has been widely used. The flux gate sensor includes a cyclic magnetic core, an exciting coil coiled around the magnetic core for applying a magnetic field, and a sensing coil for detecting the magnetic flux density of the magnetic core. Thus, it has a bulky shape and is miniaturized with great difficulty.
Another azimuth sensor uses two magnetoresistive or MR elements. These MR elements are arranged in a common plane so that paths of the currents applied to these MR elements are mutually perpendicular and connected to a bridge to detect the direction of the magnetic flux of an external magnetic field. The azimuth sensor has a simplified shape and will be easily miniaturized.
An azimuth sensor using conventional MR elements, however, has a small rate of change in inherent resistance of 3% to 6% to the intensity of the external magnetic field. Such an insensitive change is unsuitable for accurate measurement of a magnetic flux of an external magnetic field such as geomagnetism and thus to an azimuth sensor.
As a result of the trend towards high definition of CAD image information, the pitch of shadow mask holes in a display having a Braun or CRT tube (hereinafter referred to as a CRT display) has become finer. For example, a CRT display having a screen size of 14 inches has a pitch of 0.28 mm/mask. In such a high definition screen, electron beams in the CRT tube deviate from the objective lines by the effect of an external magnetic field such as geomagnetism, resulting in deterioration of image quality, e.g. distorted images, and uneven colors. Current CRT displays, therefore, have autocancelers for canceling the effect of the geomagnetism. The autocanceler has a canceling coil for applying a magnetic field having the reverse vector to the magnetic field of the geomagnetism, that is, a canceling magnetic field to the cathode ray tube, and a controller for controlling the vector of the canceling magnetic field.
A typical conventional controller for the autocanceler has a flux gate magnetic sensor having excellent stability according to its operational principle and a high sensitivity of 10.sup.-7 to 10.sup.-6 G, as in the azimuth sensor. The flux gate sensor includes a cyclic magnetic core, an exciting coil coiled around the magnetic core for applying a magnetic field, and a sensing coil for detecting the magnetic flux density of the magnetic core. Thus, it is bulky and is miniaturized with great difficulty.
Another magnetic sensor for the autocanceler uses two MR elements. These MR elements are arranged in a common plane so that paths of the currents applied to these MR elements are mutually perpendicular and connected to a bridge to detect the direction of the magnetic flux of an external magnetic field. The autocanceler has a simplified shape and will be easily miniaturized.
A magnetic sensor using conventional MR elements, however, has a small rate of change in inherent resistance of 3% to 6% to the intensity of the external magnetic field, as in the azimuth sensor. Such an insensitive change is unsuitable for accurate measurement of a magnetic flux of an external magnetic field such as geomagnetism. Thus, the vector of the canceling magnetic field for normally operating the autocanceler cannot be optimized.
Recently, further miniaturization and further improvement in recording density have been required in magnetic recording units, such as hard disk drives as external memory units, digital audio tape recorders, and digital videotape recorders. Development of high performance magnetic heads is essential for such requirements, and magnetic reproduction heads using MR elements have been developed.
Since a magnetic head having a MR element does not have a dependence on a relative velocity with respect to the recording medium, it is suitable for reading recorded signals at a low relative velocity. It has a low sensitivity to output signals because of a low change rate in response to a change in the recorded magnetization on the recording medium. Accordingly, it will be difficult to satisfy future demands for high-density recording.
Under the above-mentioned circumstances, MI elements have recently attracted attention. A typical conventional magnetic sensor of a magnetic head using the MI element will now be described with reference to the drawings. With reference to FIGS. 19A and 19B, a magnetic head 1 has a pair of cores 2a and 2b composed of ferrite as a ferromagnetic oxide, and a MI element 5 as a magnetic material. The MI element 5 is bonded to the cores 2a and 2b with a bonding glass 3 interposed therebetween. The MI element 5 is magnetically coupled with the cores 2a and 2b. That is, the ends 5a and 5b in the longitudinal direction of the MI element 5 are bonded to the magnetic circuit connecting faces 3a and 3b of the cores 2a and 2b, respectively. An insulating layer (not shown in the drawings) is formed on the magnetic circuit connecting faces 3a and 3b. The cores 2a and 2b and the MI element 5 thereby form a closed magnetic circuit.
The bonding glass 3 is composed of a nonmagnetic material, prevents direct magnetic coupling between the paired cores 2a and 2b, and is bonded to the lower faces of the cores 2a and 2b. A magnetic gap G is provided between the cores 2a and 2b. A regulating groove 4 is provided on the magnetic gap G for regulating the track width of the magnetic gap G, and filled with glass 8 as a nonmagnetic material. Conductive films composed of Cu, Au, or the like are deposited to form terminals 6a and 6b on the two ends of the MI element 5 in the longitudinal direction. The terminals 6a and 6b are each connected to a lead 7 for extracting output signals and a lead (not shown in the drawings) for applying an alternating current.
The magnetic head 1 operates as follows. An external magnetic field by the recorded magnetization on a recording medium (not shown in the drawing) invades the cores 2a and 2b through the magnetic gap G and is applied to the MI element 5. An alternating current of a MHz band has been previously applied to the MI element 5 to generate a voltage between two ends of the MI element 5 by the impedance inherent in the MI element. The amplitude of the voltage varies within a range of several tens of percent in response to the intensity of the external magnetic field and is extracted as output signals through the lead 7.
The magnetic head 1 using the MI element 5 has a significantly high change in the extracted voltage for a very weak external magnetic field of several gausses which is applied to the MI element 5 from the recording medium, hence the magnetic head 1 can have high sensitivity. Further, such high sensitivity permits reduction in the effective cross-sectional area of the magnetic flux in the magnetic circuit, and thus reduction in the size of the cores 2a and 2b, and this allows miniaturization of the magnetic head 1.
Conventional materials used for MI elements are amorphous ribbons composed of Fe--Si--B system alloys, e.g., Fe.sub.78 Si.sub.9 B.sub.13 and amorphous wires composed of Fe--Co--Si--B system alloys, e.g., (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15, as described above. As shown in FIG. 18, however, a magnetic head using a MI element composed of the Fe.sub.78 Si.sub.9 B.sub.13 alloy produce a low output voltage from the MI element for the applied external magnetic field. Thus, the output signals must be amplified by about 100 times. The element, therefore, cannot produce high quality output signals because of noise generation during the amplification.
On the other hand, a magnetic head 1 using a MI element composed of the (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 alloy produces a high voltage from the MI element for the applied external magnetic field, resulting in a low amplification factor of the output signals, as shown in FIG. 18. The output voltage, however, steeply and nonquantitatively varies within the very weak external magnetic field range from -2 Oe to +2 Oe. As a result, the MR element cannot be used as a magnetic field detecting element of the magnetic head. Although it can be used in a magnetic field region of 2 Oe or more as the absolute value, a coil must be provided to apply a considerable amount of current that is required for such a large bias magnetic field. When the bias magnetic field is applied from a permanent magnet having a magnetization of about 2 Oe, a complicated configuration of the magnetic head 1 is unavoidable.